The present invention relates to an amplifying circuit for dynamic focus for correcting dynamic focus of television receiver or CRT monitor using a cathode-ray tube.
In a television receiver, or CRT monitor, recently, the surface of the cathode-ray tube tends to be flat. At the same time, as the definition is becoming higher, the scanning becomes faster. In this situation, a higher definition of the screen is strongly demanded.
At the present, therefore, dynamic focus correction is standard in the television receiver or CRT monitor using a cathode-ray tube.
Conventional amplifying circuits for dynamic focus are described below. FIG. 7 is a block diagram of an amplifying circuit for dynamic focus in a prior art. FIG. 8 is a circuit diagram showing a practical circuit of the amplifying circuit for dynamic focus in the prior art.
In FIG. 7, an amplifying circuit 71 for horizontal dynamic focus receives, amplifies, and delivers a parabolic waveform signal for horizontal dynamic focus correction synchronized with a horizontal sync signal.
In a detection amplifier 72 for DC feedback, the output of the amplifying circuit 71 for horizontal dynamic focus is divided by using two series resistors, and entered in a non-inverting input terminal, and horizontal parabolic components are filtered by connecting a capacitor between this dividing point and the ground. On the other hand, a supply voltage +VL is divided by two series resistors, and entered in an inverting input terminal, and is connected to the ground through a capacitor from this dividing point. A differential voltage between the non-inverting input terminal and inverting input terminal is amplified, and is delivered to a DC control circuit 73.
In the DC control circuit 73, by the output from the detecting amplifier 72 for DC feedback, the DC action point of the amplifying circuit 71 for horizontal dynamic focus is controlled.
In this constitution, a DC feedback loop is constructed in order to deliver output voltage without distortion by stabilizing the DC action point of the amplifying circuit 71 for horizontal dynamic focus.
To an amplifying circuit 74 for vertical dynamic focus, consequently, a parabolic waveform signal for correcting the vertical dynamic focus synchronized with the vertical sync signal is entered and amplified, and its output voltage is applied to a coupling point of a resistor and a grounded capacitor, after the output voltage of the amplifying circuit 71 for horizontal dynamic focus passes through the capacitor for AC coupling, that is, the point of passing the low pass filter for filtering the parabolic waveform signal for correction of horizontal dynamic focus. To the parabolic waveform signal for correction of horizontal dynamic focus, the parabolic waveform signal for correction of vertical dynamic focus is added and superposed, and is applied to a grid for dynamic focus of cathode-ray tube which is not shown in the drawing.
Referring to FIG. 8, a practical circuit of amplifying circuit for dynamic focus of prior art is described. Same functional parts as in FIG. 7 are identified with same reference numerals, and explanations are omitted.
In FIG. 8, the amplifying circuit 71 for horizontal dynamic focus is mainly composed of transistors Q81, Q82, Q83, Q84, Q85. A parabolic waveform for correction of horizontal dynamic focus is entered to the base of the transistor Q83. A specific voltage VB is entered to the base of the transistor Q82. The collector of the transistor Q83 is connected to the emitter of the transistor Q82, and a cascade connection type amplifying circuit is composed. The load of the cascade type amplifying circuit is the transistor Q81 connected to the collector of the transistor Q82. The parabolic waveform signal for correction of horizontal dynamic focus amplified in this cascade type amplifying circuit is delivered from the collector of the transistor Q82, and is fed into the bases of the transistors Q84, Q85 in emitter-follower configuration for lowering the output impedance, and is delivered from combined emitters. The constitution around the detection amplifier 72 for DC feedback is same as described in FIG. 7.
The DC control circuit 73 is constructed around a transistor Q86. The output voltage of the detection amplifier 72 for DC feedback is divided by two series resistors, and is entered into the base of a transistor Q86 from the connection point of two resistors. The collector of the transistor Q86 is connected to the emitter of the transistor Q83, the collector current of the transistor Q86 is controlled by the output voltage of the detection amplifier 72 for DC feedback, and the action point of a cascade type amplifying circuit composed of transistors Q82 and Q83 is controlled by this collector current, thereby controlling the DC action point of the amplifying circuit 71 for horizontal dynamic focus.
In this constitution, a DC feedback loop is composed in order to deliver an output voltage without distortion by stabilizing the DC action point by control of the DC action point of the amplifying circuit 71 for horizontal dynamic focus.
The amplifying circuit 74 for vertical dynamic focus is composed around a transistor Q87, and a parabolic waveform signal for correction of vertical dynamic focus is entered in the base of the transistor Q87, and amplified and delivered. The output signal of The transistor Q87 which is the output of this amplifying circuit 74 for vertical dynamic focus is connected to the point passing through a capacitor for AC coupling and low pass filter from the emitters of the transistors Q84, Q85 which are the output stage of the amplifying circuit 71 for horizontal dynamic focus, and a parabolic waveform signal for correction of horizontal dynamic focus which is the output of the amplifying circuit 71 for horizontal dynamic focus, and a parabolic waveform signal for correction of vertical dynamic focus which is the output of the amplifying circuit 74 for vertical dynamic focus are added and superposed, and delivered to a grid for dynamic focus of a cathode-ray tube.
In such conventional constitution, in addition to the problem in circuit configuration that the amplifying circuit 74 for vertical dynamic focus is necessary, since the parabolic waveform signal for correction of horizontal dynamic focus and parabolic waveform signal for correction of vertical dynamic focus are amplified and added in a high voltage state of hundreds of volts, in FIG. 8 the output circuit of low output impedance of the amplifying circuit 71 for horizontal dynamic focus is a load for the amplifying circuit 74 for vertical dynamic focus. and the low pass filter is a load for the amplifying circuit 74 for vertical dynamic focus, and hence the waveform distortion is large when adding.